1. Field of the Invention
This invention is related generally to a method and apparatus for feeding waste fuels with a high solids content into a cement kiln, and is related specifically to a method and apparatus for continuously feeding hazardous waste fuels with high solids content into a cement kiln as a supplemental fuel source.
2. Prior Art
Cement is produced by heating raw materials, such as limestone and clay, at high temperatures of typically greater than 1400.degree. C. to produce nodules which, when cooled, are referred to as clinker. Clinker kiln systems typically consist of a rotary kiln tilted at a slight angle and a cyclone-type suspension preheater. More modern kiln systems are also equipped with a precalciner. The finely ground and dried raw materials are fed to the preheater for preheating using the combustion gases from the kiln. From the preheater, the hot raw materials slide into the feed end of the rotary kiln, which is the highest point of the kiln, and are slowly conducted downward by the rotation of the kiln, countercurrent to the combustion gases. Combustion takes place at the lower end of the rotary kiln where the clinker material is discharged into the clinker cooler.
The raw materials first enter the preheater in which they are typically preheated to a temperature of approximately 800.degree. C. to 850.degree. C. From the preheater the raw materials, partially calcined, are fed to the rotary kiln in which material temperatures approach 1000.degree. C. After calcination, the materials enter the burning or sintering zone, which is typically at a temperature of approximately 1450.degree. C., where the chemical reaction causing the formation of clinker occurs. The clinker then exits the kiln at the lower end into a clinker cooler which serves as a preheater for the combustion air.
Heat is produced in the burning zone by the introduction of various fuels, typically pulverized coal, gas or oil. Recently, however, pumpable and solid hazardous wastes have been used a supplemental fuel to produce the heat. The pumpable hazardous wastes are fed to the burner with the sintering zone in a similar way as oil has been fed. For introducing solid hazardous waste fuels into the rotary kilns, various methods have been developed. In cement kilns with suspension preheaters burning solid hazardous waste fuel, the solid hazardous waste fuel is metered typically in batch quantities into the rotary kiln, such as in six gallon pails or in smaller pails. The prior art describes how the solid hazardous waste fuel charges are introduced directly into the combustion zone of the rotary kiln, or into the feed end housing of the rotary kiln where also the preheated raw meal is fed to the kiln. However, this type of batch or charge feed does not produce the desired constant heat source produced from a continuous feed.
In U.S. Pat. No. 3,572,524 to Muckenheim, an apparatus for charging sludges and other similar waste materials to the feed end of a rotary incinerating kiln using an endless screw-conveyor is disclosed. The Muckenheim apparatus is particularly suitable for use in conjunction with charging sludges, viscous refuse, pastey refuse, and semifluid refuse to a rotary kiln. The refuse is dumped into a stationary receiving trough having a rotatable, circular distribution plate located above the bottom of the trough. A horizontal screw-conveyor is disposed radially to the distribution plate, protruding into the receiving trough at one end and terminating at the other end in the feed housing of the rotary kiln. Refuse dumped into the receiving trough settles onto the distribution plate which, as it rotates, causes the refuse to contact the screw-conveyor, which withdraws the refuse from the trough and feeds it into the feed housing of the rotary kiln.
One disadvantage of the Muckenheim apparatus is that the screw-conveyor can only transmit the waste into the feed housing of the kiln, and cannot develop the pressure head required to pump the waste through a long pipe if it is desired to introduce the waste to a point within the rotary kiln itself. Additionally, the screw-conveyor introduces the refuse to the feed housing of the rotary kiln where the refuse first mixes with the preheated raw material prior to entering the rotary kiln. Such premature introduction of the fuel source to the preheated raw material may result in inefficient combustion and clinker product of unknown quality. Further, the use of such a rotatable distribution plate may allow the refuse to build up along the inner wall surface of the receiving trough, resulting in less efficient operation of the feed device. Furthermore, the screw conveyor would be exposed to 800.degree. C. to 850.degree. C. material temperatures and 900.degree. C. to 1000.degree. C. gas temperatures in the feed end housing of a cement kiln with suspension preheater, and no screw conveyor material is known which could withstand such temperatures.
In U.S. Pat. No. 4,850,290 to Benoit et al., a method for charging drums of solid hazardous waste directly into the central portion of a rotary kiln or into the feed end housing of a kiln with suspension preheater is disclosed. In its best mode, the Benoit apparatus comprises a feed tube located on the rotary kiln itself which, as the kiln rotates, cooperates with a second stationary feed device located beside the rotary kiln. Drums of hazardous waste are placed within the stationary feed device, and as the feed tube on the kiln itself rotates by the stationary feed device, the drums are picked up by the rotating tube. When the rotating tube reaches the eleven to one o'clock position, a slide gate opens and the drum falls into the rotary kiln. Alternately, the Benoit patent describes a chute with double air locks through which the six gallon pails can be fed into the feed end housing of a kiln system with a preheater. The primary disadvantage of the Benoit method is its batch feed operation which can cause spikes of combustibles and temperature within the rotary kiln and in the feed end housing. Another disadvantage of this method is the need to use constant size hazardous waste containing drums as the feed. This results in varying heat content of the pails as the waste composition and, consequently, the heat content of the waste changes. Consequently, the Benoit method results in uneven combustion, changing from low excess oxygen or even insufficient oxygen to too much oxygen for the combustion process.
In U.S. Pat. No. 4,984,983 to Enkegaard, a method of cofiring hazardous waste in industrial rotary kilns is disclosed. In effect, the Enkegaard method launches charges of hazardous waste directly into the burning zone of a rotary kiln using a cannon-like apparatus. The hazardous waste is placed within the cannon-like apparatus which, using a charge such as compressed air, ejects the hazardous waste pails directly into the burning zone of the kiln. Similar to the Benoit apparatus, the Enkegaard apparatus suffers the disadvantage of batch-like feeding resulting in uneven combustion conditions.
Current cement kiln feeding technology of hazardous wastes as fuel can be categorized as follows: (1) continuous feed of liquid, pumpable wastes with up to 40% suspended solids to the hot or discharge end; or (2) batch feed of high solids content wastes to the feed end housing or mid-kiln. Although the batch feed technology is applicable to both high liquids content and high solids content wastes, current continuous feed technology cannot be applied to high solids content wastes. Therefore, it can be seen that there is a need for a technology for the continuous feed of high solids content hazardous waste fuels to rotary cement kilns. It is to this end that the present invention is directed.